How to Check Leakage with a Digital Multimeter
When one probe touches the metal plating on the power socket and the hand touches the metal part of the other probe, the meter displays a very clear voltage value of 119V! The metal coating on the socket is indeed charged. After investigation, it was found that the plastic protective layer of the live wire was broken by the fixing screw of the socket, which came into contact with the californium core. The screw was pressed against the metal coating, causing the socket to become electrified. The internal resistance of the voltage range of a digital meter is usually 10 M Ω. The connection between the human body and the ground can be equivalent to a resistor (a series resistor l for shoes and wooden flooring) connected in parallel with a capacitor (a distributed capacitor between the human body and the ground), as shown in the attached figure. The voltage on the live wire is transmitted to the ground through probe l, the electric meter, probe 2, the DC resistance of the human body and the wooden board, and the distributed capacitance of the human body connected in parallel. The electric meter displays the internal resistance at both ends of the meter, where UDY is the voltage from the live wire to the ground, RB is the internal resistance of the voltmeter, and Z is the total impedance between the human body and the ground. Obviously, the larger the internal resistance of the electricity meter, the higher the sensitivity of the electrical verification test. It must be pointed out that the above electrical verification method requires a current to pass through the human body, and only the electric meter can indicate it. Undoubtedly, it should be small enough, so small that the human body cannot feel the presence of the current. A digital multimeter has an internal resistance of 100M Ω. Even if standing barefoot on a damp ground, the effective value of the current flowing through the human body is only 22 μ A when the live voltage is 220V. A pointer type multimeter is generally not suitable for electricity testing because the voltage range resistance of such meters is much less than 10 M Ω. Therefore, the sensitivity of electrical testing is very low. Attempting to increase the sensitivity of electrical testing using a low voltage range is incorrect, as low voltage ranges typically have low internal resistance (the internal resistance of a pointer multimeter voltage range is the product of the ohm value per volt multiplied by the voltage range). The sensitivity of the indication is almost not improved, and instead, it is prone to electric shock accidents.
